Lithographic printing plates which are now generally used are obtained by submitting so-called presensitized printing plates (hereinafter referred to as "PS plates") to plate making processing. The PS plates are prepared by coating light-sensitive materials on aluminum plates having surfaces subjected to surface treatments, e.g., a graining treatment, an oxidizing treatment, and so on, and drying them. The plate making process includes an imagewise exposing step, a developing step, a step of washing with water, a step of treating with lacquer and so on. It is well known that the areas where the light-sensitive layer remains undissolved after the above-described developing step form the image part and the areas where the light-sensitive layer is removed to expose the underlying aluminum surface become water-acceptable. Accordingly, such water-acceptable portions form the non-image part.
Aluminum plates are generally used as the supports of such lithographic printing plates. Conventional materials used for this purpose are prepared from aluminum alloy plates produced by casting aluminum alloys, e.g., JIS A3003, JIS A1100, JIS A1050, etc., into ingots using a conventional continuous casting process. The ingots are then submitted to a hot rolling process and a cold rolling process and optionally a heat treatment at an intermediate stage during the rolling processes. The thus-prepared aluminum alloy plates are then subjected to a surface roughening treatment using a mechanical, chemical or electrochemical technique and subsequently to an anodic oxidation.
More specifically, in accordance with known methods, aluminum plates are subjected in sequence to a mechanical roughening treatment, a chemical etching treatment and anodically oxidized film forming processing, as described in U.S. Pat. No. 3,834,998. According to another process, plates are subjected to a chemical etching treatment and subsequently to anodically oxidized film forming processing, as described in Japanese Patent Application (OPI) No. 61304/76 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"). In another process, aluminum plates are subjected in sequence to an electrochemical treatment, an after-treatment and anodically oxidized film forming processing, as described in Japanese Patent Application (OPI) No. 146234/79. In yet another process, aluminum plates are subjected in sequence to an electrochemical treatment, a chemical etching treatment and anodically oxidized film forming processing, as described in Japanese Patent Publication No. 28123/73. In still another process, aluminum plates are subjected to a mechanical roughening treatment and subsequently to the treatments described in Japanese Patent Publication No. 28123/73.
However, the use of conventional aluminum alloy plates may generate dot-form defects upon both the chemical etching and the electrochemical etching treatment causing stain in prints obtained. Accordingly, they have turned out to be disadvantageous with respect to obtaining high print quality.
In recent times, printing speeds have been increased. Accordingly, this causes an increase in the stress applied to the printing plate which is fixed mechanically at both ends of a rotating cylinder installed in a printing machine. When put under such stress, aluminum alloy plates often have insufficient mechanical strengths. Accordingly, deformation or breaks of the printing plate at the above-described fixed parts result. A breakdown of the printing plate makes it impossible to continue the printing operation.
Previously, relatively thick aluminum alloy plates had to be employed to ensure sufficient mechanical strength such as dimensional stability. The need for thicker plates was a major factor increasing manufacturing costs of lithographic printing plates.